The curves of well-defined abdominal muscles that start at the lateral iliac crests and end medially at the pubic tubercle. They form the "V" that outlines a 6-pack. They are referred to as such both because they can be worked out during sexual activity, and possession of such musculature makes you want to have sex with that person.

Did you see his sex muscles when he took his shirt off? I think i just peed my pants a little.